Modern aircraft, mainly large size ones, normally contain an auxiliary power unit (APU) comprising a power module (engine) supplied with fuel, a pneumatic compressor, and/or a hydraulic pump, a gearbox and one or several electric generators, in order to provide electric, pneumatic or hydraulic power, or a combination thereof. The main use of the APU is intended for aircraft when they are on the ground, with the main motors switched off, and the aircraft cannot be supplied with the electric, pneumatic or hydraulic external power necessary for the desired use of it.
The power source that is most easily available in airports nowadays is electric power. On the other hand, the most difficult power source to be available is high-pressure pneumatic power. When the aircraft is on the ground and the environmental conditions are at their most extreme, due to very cold or hot temperatures with or without the combination of high moisture, it is more necessary to fit the cabin (the cabin being understood as the area that has to be kept pressurized in flight) for the comfort of the passengers, crew and operators, with the consequent increase in the demand for power, and the use of the APU becomes habitual and more critical on account of the shortcoming mentioned above.
The conventional arrangement described above presents the drawback of the noise produced, and the fuel consumed during its functioning, which produces acoustic contamination and air pollution.
This mode of functioning permits complete autonomy of the aircraft but it implies a considerable operating cost for the airlines. In addition, it is expected that greater restrictions or penalties will exist in the future on the use of the APU, as a result of national, international or airport regulations, for acoustic and/or environmental reasons.